1. Field of the Invention
The present invention relates to an ultrasonic imaging apparatus using an ultrasonic transducer constituted by multi-layer piezoelectric elements.
2. Description of the Related Art
An ultrasonic transducer is constituted by piezoelectric elements. The ultrasonic transducer generates ultrasonic waves so that the internal state of an object is inspected by using the reflected ultrasonic waves. The ultrasonic transducer is used for diagnosis of the inside of a human body, flaw detection of the inside of a welded metal, and the like. An array type ultrasonic transducer constituted by an array of a plurality of piezoelectric elements is widely used.
A medical ultrasonic imaging apparatus is mainly used as an ultrasonic diagnosis apparatus for obtaining a tomographic image (B mode image) of the inside of a human body, i.e., the abdomen, further, can be used to form, in addition to a tomographic image, a so-called Doppler mode image for observing a blood flow rate in a heart, a carotid artery, or the like by utilizing the Doppler effect. Furthermore, in this ultrasonic diagnosis apparatus, color display of the blood, i.e., color mapping can be realized.
However, a sensitivity margin of the ultrasonic diagnosis apparatus including an ultrasonic transducer in observation of the blood is smaller than that in formation of a B mode image. This is based on the fact that a method of obtaining a signal using the Doppler mode is different from a method of obtaining a B mode image. Therefore, as the sensitivity of the ultrasonic transducer is increased, the quality of a Doppler mode image is noticeably improved compared with a B mode image.
Four methods of increasing the sensitivity of the ultrasonic transducer are available, namely, (1) an increase in a drive voltage, (2) improvement of an piezoelectric material, (3) acoustic matching, and (4) electrical matching. With regard to method (1), since the number of elements tends to be increased in recent or future ultrasonic transducers, most drive sources are constituted by hybrid ICs, and hence, it is difficult to apply a high drive voltage to an ultrasonic element. With regard to method (2), electromechanical coupling coefficiency k.sub.33 of a piezoelectric ceramic material is about 0.7. In order to double ultrasonic sensitivity of the ultrasonic transducer by increasing the electromechanical coupling coefficiency, the value of k.sub.33 must be set to be about 0.95. However, it is practically impossible to realize this value. With regard to method (3), since ultrasonic sensitivity and resolution generally conflict with each other, a great increase in ultrasonic sensitivity by acoustic matching cannot be expected without loss of resolution. In contrast to methods (1) to (3), according to method (4), ultrasonic sensitivity can be effectively increased because of the following reasons.
In the Doppler mode, an electronic sector scan type ultrasonic transducer is used. When the Doppler mode is to be executed, an ultrasonic beam must be obliquely radiated onto a blood vessel to be observed. In order to prevent grating robe due to this oblique radiation, the electronic sector scan type ultrasonic transducer is used because it has an element pitch smaller than that of an electronic linear scan type ultrasonic transducer, and hence, more suitable for the purpose.
The area of one element of the electronic sector scan type ultrasonic transducer is 1/2 to 1/4 that of the electronic linear scan type ultrasonic transducer. For this reason, an impedance per element of the electronic sector type ultrasonic transducer is larger than that of the electronic linear scan type ultrasonic transducer. If the impedance of an ultrasonic transducer element is large, the voltage loss of a reflected wave signal obtained from the transducer element occurs because of the electrostatic capacity of a coaxial cable connecting the ultrasonic transducer to a receiver section and/or the input impedance of the receiver section.
That is, the voltage of the reflected wave signal is considered to be determined by the ratio of the parallel combined impedance of the impedance determined by a cable capacitance and the input impedance of the receiver section, to the serial combined impedance of this parallel impedance and the impedance of the ultrasonic transducer. Hence, the higher the impedance of the ultrasonic transducers, the greater the voltage loss of the reflected wave signal. The imaging apparatus can be more sensitive to the ultrasonic waves if the impedance of the ultrasonic transducer is reduced. To reduce the impedance of the transducer, the following methods can be used.
The first method is to incorporate a piezoelectric element having a great dielectric constant into the transducer. However, the greatest relative dielectric constant that a piezoelectric element can have is 5000. Further, the greater the dielectric constant, the smaller the coupling coefficiency of the piezoelectric element, and the lower the Curie temperature.
According to the second method, an impedance converting means such as a coil, a transformer, or an FET is used. In this method, if the impedance converting means is incorporated in a head section of an ultrasonic transducer having several tens of elements or 100 or more elements, the size of the ultrasonic transducer is increased, resulting in degradation in operability of the ultrasonic transducer. In addition, since the impedance converting means has predetermined frequency characteristics, the operating band of the ultrasonic transducer is narrowed.